Mark Hughes is expected to leave Manchester City with a maximum £3m pay-off under the contract granted by the former owner Thaksin Shinawatra as he had not received an improved deal from the club's oil-rich backers in Abu Dhabi.

Despite being entrusted with a £200m overhaul of City's playing staff, Hughes's terms never improved on the three-year deal worth an estimated £35,000-£40,000 a week he signed on joining from Blackburn Rovers in June 2008. His arrival came two months before Sheikh Mansour purchased City from the controversial former Thai prime minister Thaksin and he leaves with 18 months remaining on that deal. The final severance figure, however, may be determined by whether he gains alternative employment in that period.

The former City manager, sacked and replaced by the former Internazionale coach Roberto Mancini with immediate effect on Saturday night, issued a staunch defence of his tenure today in which he accused the club's owners of shifting previously agreed targets for this season.

Mancini will be unveiled as City's new manager at Eastlands tomorrow, having agreed a three-and-a-half-year contract and with many of Hughes's signings for the club – such as Craig Bellamy, Gareth Barry and Shay Given dismayed by his sacking. The three were part of a delegation that urged the club's chief executive, Garry Cook, to reverse the decision on Saturday evening.

In a statement issued by the League Managers Association today, Hughes denied knowing Saturday's 4-3 win over Sunderland was to be his last game as City manager and claimed to have been betrayed. It read: "I was informed after yesterday's match against Sunderland that my contract with Manchester City was being terminated with immediate effect. Notwithstanding media coverage to the contrary, I was given no forewarning as to the club's decision. Given the speed with which my successor's appointment was announced, it would appear that the club had made its decision some considerable time ago."

Hughes also believes City had altered their ambitions during the course of this season and reneged on a previous understanding. The former Wales manager added: "At the beginning of the season I sat down with the owners and it was agreed that a realistic target for the season would be sixth place in the Barclays Premier League, or in the region of 70 points. All of this was communicated to the players and we all knew where we stood. Whilst everyone at the club would obviously have wanted to see more wins, we were absolutely on target at the time of my dismissal. Only recently we had terrific victories against both Arsenal and Chelsea."

The City chairman, Khaldoon al-Mubarak, said the decision to sack Hughes was in response to a run of just two wins in 11 league matches. However, Guus Hiddink's agent has claimed there was contact from City's chief executive, Garry Cook, earlier this season and there are unconfirmed reports that the Liverpool manager, Rafael Benítez, was also sounded out about replacing Hughes before the campaign began.